With the wide-spread application of identification on the basis of fingerprints, the requirements imposed on a fingerprint reader apparatus and especially on the objective lens system thereof are diverse. It is desired to produce such objectives simply and at a low cost. In addition, the optical image of the fingerprint should be projected by the objective lens system to the image detector with an image quality corresponding to the physical resolution of the image detector applied. From the aspect of low cost production, it is also desirable that the objective lens system allows simple adaptation to various print area sizes, the latter basically depending on the space limited by the size of the fingerprint reader apparatus and on the desired reliability of fingerprint identification.
In a fingerprint reader apparatus, a total-reflection prism well known per se is generally used for displaying the fingerprint as an object, while the imaging of the object is carried out by an objective lens system by means of telecentric, central or afocal ray path imaging.
For example, in EP 0 308 162 A2 a fingerprint imaging optical system is described. In this known system, the image of a fingerprint appearing on the print area having a size of 19 mm×19 mm is telecentrically projected to the image detector via a distortion reducing correcting prism, a lens system consisting of two aspherical lenses, and an aperture stop located after the lens system. Because of the arrangement used therein, the optical system requires a very long optical path, which would result in large overall dimensions. To avoid this, auxiliary mirrors are arranged in the ray path, to ensure a still acceptable size of the system. The image created by this optical system is of an acceptable quality with limited distortion, thanks to the compensation prism applied.
In U.S. Pat. No. 5,764,347 an optical imaging system is described, where light beams exiting from a total-reflection prism formed with a special light exit surface are imaged by an optical unit consisting of three or four spherical lenses to an image-surface arranged at a distance from the optical axis. In the imaging system, the print area has a size of 25 mm×25 mm. In this known optical imaging system, a central ray path imaging is applied and a relatively high quality imaging is achieved with moderate distortion. The special design of the prism and the special arrangement of optical elements, however, make this optical system expensive to produce.
In JP 8 334 691 a generally distortion-free afocal imaging is described, where the print area has a size of approximately 20 mm×30 mm, and the objective lens system consists of two pairs of spherical lenses and an aperture stop between them. This design is relatively costly due to its complexity. In the fingerprint reader apparatus described in JP 10 269 344, an objective lens system with a pinhole aperture stop is used, which results in a moderate image quality, and an objective lens system consisting of two lenses is utilised for afocal imaging. It is the common disadvantage of these optical systems that due to the application of afocal imaging, adversely large overall dimensions are necessitated.
In U.S. Pat. No. 5,900,993 optical systems for fingerprint reading are described, where the objective lens system comprises a first lens guiding light beams arriving from a print area telecentrically, an aperture stop with a relatively large diameter located in the focal point of the first lens and a correcting lens system consisting of lenses arranged at both sides of the aperture stop. This correcting lens system—in order to reduce imaging disadvantages resulting from the relatively large aperture stop—is formed in a relatively complex way. The correcting lens systems of the embodiments disclosed in the specification basically consist of one lens having at least one aspherical surface arranged at each side of the aperture stop, except for the embodiment depicted in FIG. 4, where lenses having exclusively spherical surfaces are used, but in which the correcting lens system consists of three lenses. The lens located on the side of the aperture stop facing the fist lens is a dispersing lens, which is used primarily for correcting field curvature caused by the other lenses. The telecentric imaging enables a relatively shorter optical path, but the use of aspherical lenses and the correcting lens system consisting of three lenses increase the production costs of these optical systems.
A further common disadvantage of the objective lens systems described above is that in the case of systems having various print area sizes, they do not make possible to provide a product range based on a uniform optical block and therefore manufactured at a relatively favourable price.